Resin-fiber composite materials are utilized in a variety of applications including the aerospace industry, for example. Structures which are constructed of resin-fiber composite materials may be exposed to heat, which may affect the composite materials in various ways. These heat-induced effects may include chemical degradation in which changes such as oxidation, material loss and the breaking and/or forming of chemical bonds occurs in the polymer chemical structure of the composite materials. Resin decomposition, charring and fiber decomposition of the composite materials may occur at increasing temperatures.
Repair or removal of heat-affected resin-fiber composite materials on a structure may involve first determining the degree of harmful heat effect to the composite substrate. Although determining the degree of heat effect to composite materials may be performed by visual inspection, heat effect to resin-fiber composite materials may not be visually apparent. Current methods of determining the presence and extent of heat effect in resin-fiber composite materials includes obtaining a series of infrared spectra of a series of heat-affected composite standards and making a thermal effect multivariate calibration model with the IR spectra and the thermal effect information from the standards. An infrared spectrum obtained from the composite material the heat effect of which is in question can then be predicted by the thermal effect model to determine the presence and assess the degree of thermal effect in the composite material.
Calibration of infrared sensors to residue strength in composite materials correlates the resin condition as read from the infrared spectra to the residual strength of the material which degrades as the resin degrades with progressively increasing temperatures. Therefore, the infrared spectroscopy sensors may be calibrated using time-controlled thermal soak standards which are obtained by exposing various composite material controls to various temperatures for a particular time period such as one hour, for example. One method of preparing the standards includes placing the standards in an oven which is calibrated periodically and monitored continuously. Thermal effect to composite materials often does NOT follow a linear course and indeed is usually a combination of overlapping degradation mechanisms in the composite resin material. Moreover, the calibration method may utilize the entire area of the oven cavity rather than the particular area in which the composite standard is confined during heating of the standard. In application, oven temperature readings may be off by 25 F. Therefore, temperature sensors with a calibrated meter may be used to read and verify correct oven temperatures +/−1 degree F. The oven temperature may be set high or low to achieve the CORRECT reading per the temperature sensors.
Therefore, a method of making thermal effect standards for composite materials which takes into account the total thermal experience of the composite standard in terms of both time and temperature is needed.